The biologically active agents including hormones that promote cyclic GMP accumulation are being identified and the relationship of the alteration in cellular cyclic GMP to the change induced in cellular function evaluated in several ways. The cellular components with which cyclic GMP interacts are being characterized. Studies are also being conducted to determine the regulatory nature of he calcium dependent phosphodiesterase activating factor with regard to its action on cyclic GMP and cyclic AMP phosphodiesterases.